ch02 plate tectonics

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Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 2: The Way the Earth Works: Plate Tectonics

German meteorologist and polar explorer. Wrote The Origins of the Continents and Oceans in 1915.

He hypothesized a former supercontinent, Pangaea.

He suggested that land masses slowly move (continentaldrift).

These were based on strong evidence.

“Fit” of the continents

Glacial deposits far from polar regions

Paleoclimatic belts

Distribution of fossils Matching geologic units

Alfred Wegener

Fig. 2.1a




Wegener ’s idea was the basis of a scientific revolution.

Earth continually changes.

Continents move, split apart, and recombine.

Ocean basins open and close.

His hypothesis was met with strong resistance:

“What force could possibly be great enough

to move the immense mass of a continent?”

Plate Tectonics

Fig. 2.1b




The scientific revolution began in 1960. Harry Hess (Princeton) proposed sea-floor spreading.

As continents drift apart, new ocean floor forms between.

Continents converge when ocean floor sinks into the interior.

By 1968, a complete model had been developed.

Continental drift, sea-floor spreading, and subduction.

Earth’s lithosphere is broken into ~20 plates that interact.

Plate Tectonics

Fig. 2.10




Evidence of Late Paleozoic glaciers found on fivecontinents.

Some of this evidence is now far from the poles.

These glaciers could not be explained unless thecontinents had moved.

Glacial Evidence

Present day

Pangaeareconstruction

Striation

Fig. 2.2a




Placing Pangaea over the Late Paleozoic South Pole: Wegener predicted rocks defining Pangea climate belts.

Tropical coals

Tropical reefs

Subtropical deserts

Subtropical evaporites

Paleoclimatic Evidence

Fig. 2.2b




Fossil Evidence

Identical fossils found on widely separated land masses. Mesosaurus—a freshwater reptile

Glossopteris—a subpolar plant with heavy seeds

Fig. 2.2c




Identical fossils found on widely separated land. Lystrosaurus—A nonswimming, land-dwelling reptile.

Cynognathus—A nonswimming, land-dwelling mammal-

like reptile.

These organisms could not

have crossed an ocean.

Pangaea explains the

distribution.

Fossil Evidence

Fig. 2.2c




Distinctive rock assemblages and mountain belts matchacross the Atlantic.

Matching Geologic Units

Fig. 2.3a

Fig. 2.3b




Wegener had multiple lines of strong evidence. Yet, his idea was debated, ridiculed, and ignored. WHY?

He couldn’t explain how or why continents moved.

Wegener died in 1930 on a Greenland expedition.

Over the next three decades, new research, new technology,

and new evidence from the oceans revived his hypothesis.

Criticisms of Wegener ’s Ideas

Evidence from beneath the sea was key to proving that Alfred Wegener ’s ideas were correct.




Earth’s Magnetic Field

Flow in the liquid outer core creates the magnetic field. It is similar to the field produced by a bar magnet.

The magnetic pole is tilted ~11.5° from the axis of rotation.

Fig. 2.4a




Fig. 2.4d

Curved field lines cause a magnetic needle to tilt. Angle between magnetic field line and surface of the

Earth is called inclination. It depends on:

Latitude

The Earth’s Magnetic Field




Rock magnetism can be measured in the laboratory. The study of fossil magnetism is called paleomagnetism.

Iron (Fe) minerals in rock preserve information about themagnetic field at the time the rocks formed.

Declination and inclination preserved in rocks often vary

from present latitude / longitude.

Instruments used in paleomagnetism

record changes in position.

These data are used to trace

continental drift.

Paleomagnetism

Fig. 2.5a




Iron minerals archive the magnetic signal at formation. Hot magma

High Temp—no magnetization

Thermal energy of atoms is very high.

Magnetic dipoles are randomly oriented.

Paleomagnetism

Fig. 2.5b




Iron minerals archive the magnetic signal at formation. Cooled magma

Low Temp—permanent magnetization

Thermal energy of atoms slows.

Dipoles align with Earth’s magnetic field.

Magnetic dipoles become frozen in alignment with field.

Paleomagnetism

Fig. 2.5b




Polar Wandering

Layered basalts record magnetic changes over time.

Inclination and declination indicate change in position.

Fig. 2.6a




Apparent Polar Wandering

Polar wandering paths were initially misinterpreted: Not the signature of a wandering pole on a fixed continent

The signature of a fixed pole on a wandering continent

Fig. 2.6b




Before World War II, we knew little about the sea floor. Echo-sounding (sonar) allowed rapid sea-floor mapping.

Sea-floor maps created by ships crossing the oceans.

Bathymetric maps are now produced using satellite data.

Sea-Floor Bathymetry

Fig. 2.7a




Oceanographers were surprised to discover that: A mid-ocean mountain range runs through every ocean.

Deep-ocean trenches occur near volcanic island chains.

Submarine volcanoes poke up from the ocean floor.

Huge fracture zones segment the mid-ocean ridge.

These observations

are all explained by

plate tectonics.

The Ocean Floor

Fig. 2.7b




The Ocean Floor

Sonar mapping delineated bathymetric features. Mid-ocean ridges

Deep-ocean trenches

Volcanic islands

Seamounts

Fracture zones

Fig. 2.8a, b




The Ocean Floor

Today’s view of the ocean floor reveals the location of:

Mid-ocean ridges

Deep-ocean trenches

Oceanic fracture zones

Fig. 2.8a




The Oceanic Crust

Earthquakes occur in distinct belts in oceanic regions. The earthquakes were surprising. They were limited to:

Parts of oceanic fracture zones

Mid-ocean ridge axes

Deep ocean trenches

Geologists realized thatearthquakes defined

zones of movement.

Fig. 2.9




Fig. 2.10

Sea-Floor Spreading

In 1960, Harry Hess published his“Essay in Geopoetry.

”

Sediment thickens away from ridges.

Earthquakes at mid-ocean ridges indicate cracking.

Cracked crust splits apart.

High heat flow from molten rock rises into the cracked crust.

New ocean floor forming at mid-ocean ridges.




Fig. 2.10

Sea-Floor Spreading

Hess called his theory“sea-floor spreading.

”

Upwelling magma erupts at the mid-ocean ridges.

New crust moves away from ridges, gathering sediment.

At trenches, the sea-floor sinks back into the mantle.

Instantly provided a mechanism for continental drift.

Continents move apart as sea-floor spreading occurs.

Continents move together as sea-floor sinks into mantle.




Evidence of Sea-Floor Spreading

Magnetism in sea-floor rocks varies farther from MOR. Stripes of positive (stronger) and negative (weaker)

magnetic intensity

Recorded in sea-floor basalts

Fig. 2.11a, b




Evidence of Sea-Floor Spreading

Magnetic anomalies map as stripes of positive andnegative intensity.

Magnetic stripes form a pattern.

The pattern is symmetric on

either side of the MOR.

Fig. 2.11c




Magnetic Reversals

Layered lava flows reveal reversals in magnetic polarity. The magnetic field sometimes “flips”; we don’t know why.

A reversed N magnetic pole is near the S geographic pole.

Reversals are geologically rapid, expressed worldwide.

Can be used as time markers.

Fig. 2.12a, b, c




Isotopic dating gives the timing of polarity reversals. A magnetic reversal time scale has been assembled.

Reversals occur at uneven intervals.

Longer intervals (500 to 700+ Ka) are called chrons.

Shorter intervals (~200 Ka) are subchrons.

Chrons for the last 4.5 Ma are named for

scientists.

Magnetic Reversals

Fig. 2.12d




Polarity reversals explain magnetic anomaly stripes. Positive anomaly—sea-floor rock normal polarity.

Negative anomaly—sea-floor rock reversed polarity.

Magnetic anomalies are symmetric across the MOR.

Sea-Floor Spreading

Fig. 2.13a, b




Sea-Floor Spreading

Sea-floor spreading explains the stripes. Magnetic polarity reversals are imprinted in sea-floor rock

as the sea floor continues to spread.

The width of the magnetic anomaly stripes:

Is related to the spreading rate

Faster spreading = wide stripes

Slower spreading = narrow stripes

Fig. 2.13c, d




Plate Tectonics Plate tectonics: the explanation of “how Earth works.”

Earth’s outer shell is broken into rigid plates that move.

Plate motion defines three types of plate boundaries

It provides a unified mechanism explaining:

The distribution of earthquakes and volcanoes.

Changes in past positions of continents and ocean basins. The origins of mountain belts and seamount chains.

The origin and ages of ocean basins

Geology at a Glance




Lithosphere

Tectonic plates are fragments of lithosphere. Lithosphere is made of both crust and the upper mantle.

The lithosphere is in motion over the asthenosphere.

Lithosphere bends elastically when loaded.

Asthenosphere flows plastically when loaded.

Fig. 2.14a




Two Types of Lithosphere

Continental: ~150 km thick. Felsic to intermediate crustal rocks

25–70 km thick.

Lighter (less dense).

More buoyant—floats higher.

Oceanic: ~100 km thick. Mafic crust: basalt & gabbro

7–10 km thick.

Heavier (more dense).

Less buoyant—sinks lower.Fig. 2.14b




Plate Boundaries

Lithosphere is fragmented into ~12 major tectonic plates. Plates move continuously at a rate of 1–15 cm/year.

Slow on a human time scale; extremely rapid geologically.

Plates interact along their boundaries.

Fig. 2.15a




Locations on Earth where tectonic plates meet. Identified by concentrations of earthquakes.

Associated with many other dynamic phenomena.

Plate interiors are almost earthquake-free.

Plate Boundaries

Fig. 2.15b




Plate Boundaries

Tectonic plates: Display a variety of sizes and shapes.

Change size and shape throughout their history.

Fig. 2.15c




Continental Margins

Where land meets the ocean. Margins near plate boundaries are “active.”

Margins far from plate boundaries are “passive.”

Earthquakes common along active margins.

Passive-margin continental crust thins seaward.

Traps eroded sediment.

Develops into the

continental shelf.

Fig. 2.14b




Plate Boundaries: Three Types

Divergent boundary—tectonic plates move apart. Lithosphere thickens away from the ridge axis.

New lithosphere created at divergent boundary

Also called: mid-ocean ridge, ridge.

Fig. 2.16a





Convergent boundary—tectonic plates move together. The process of plate consumption is called subduction.

Also called: convergent margin, subduction zone, trench.

Fig. 2.16b





Transform boundary—tectonic plates slide sideways. Plate material is neither created nor destroyed.

Also called: transform fault, transform.

Fig. 2.16c




Sea-floor spreading progression. Early stage

Rifting has progressed to mid-ocean ridge formation.

Before substantial widening of the ocean.

Forms a long, thin ocean basin with young oceanic crust.

Example: The Red Sea

Divergent Boundaries

Note: This diagram depicts only the crust, not the entire lithosphere.

Time 1

Youngest

Ocean Floor

Fig. 2.17a




Divergent Boundaries

Sea-floor spreading progression. Late stage

Mature, wide ocean basin.

Linear increase in age with distance from central ridge.

Edge of ocean basin—oldest; ridge proximal—youngest.

Example: The Atlantic Ocean

Note: This diagram only depicts the crust, not the entire lithosphere.Fig. 2.17a

Time 3




Mid-Ocean Ridges

Linear mountain ranges in Earth’s ocean basins.

Example: The Mid-Atlantic Ridge

Snakes N–S through the entire Atlantic Ocean.

Elevated ridge (1,500 km wide) 2 km above abyssal plains.

New sea floor created only along axis of the ridge

Symmetrical

Fig. 2.17b




Mid-Ocean Ridges

Sea-floor spreading opens the axial rift valley. Rising asthenosphere melts, forming mafic magma.

Pooled magma solidifies into oceanic crustal rock.

Pillow basalt—magma quenched at the sea-floor.

Dikes—preserved magma conduits.

Gabbro—deeper magma.

Fig. 2.17c




Ocean Crustal Age

Oceanic crust spreads away from the ridge axis. New crust is closer to the ridge; older crust farther away.

Oldest oceanic crust is found at the far edge of the basin.

Fig. 2.19




The hot asthenosphere is at the base of the MOR. Aging ocean crust moves away from this heat:

Cooling, increasing in density and sinking.

Older, thicker lithosphere sinks deeper into mantle.

Oceanic Lithosphere

Fig. 2.20a, b




Convergent Boundaries Lithospheric plates move toward one another.

One plate sinks back into the mantle (subduction).

The subducting plate is always oceanic lithosphere.

Continental crust cannot be subducted—too buoyant.

Subduction recycles oceanic lithosphere.

Subduction is balanced by sea-floor spreading.

Earth maintains a constant

circumference.

Convergent boundaries also

called Subduction Zones.

Fig. 2.16b




Subduction Old oceanic lithosphere is more dense than mantle.

A flat-lying oceanic plate doesn’t subduct easily.

Plate edge bends down and slips into mantle, then theleading edge sinks downward like an anchor rope.

Fig. 2.21a




Convergent Boundaries

The subducting plate descends at an average of 45°.

Plate descent is revealed by Wadati-Benioff earthquakes.

Earthquakes deepen away from trench.

Quakes cease below 660 km.

Plate descent may continue

past the earthquake limit.

The lower mantle may be

a “plate graveyard.”

Fig. 2.21b




Subduction Features

Subduction is associated with unique features:

Deep-ocean trenches.

Accretionary prisms.

Volcanic arcs.

Back-arc basins.

Fig. 2.21c





Volcanic Arc—a chain of volcanoes on overriding plate.

The descending plate partially melts at ~150 km depth.

Magmas rise and melt through overriding plate.

Arc type depends upon the overriding plate.

Continental crust—continental arc.

Oceanic crust—island arc.

Fig. 2.21d




Back-arc basins—a marginal sea behind an arc.

Forms between an island arc and a continent.

Offshore subduction traps a piece of oceanic crust, or

Stretching lithosphere creates a new spreading ridge.


Fig. 2.21e




Transform Boundaries

Lithosphere fractures and slides laterally

No new plate forms; none consumed.

Many transforms offset spreading ridge segments.

Some transforms cut through continental crust.

Characterized by:

Earthquakes

Absence of volcanism

Fig. 2.22a




Oceanic Transforms

The mid-ocean ridge axis is offset by transform faults.

Fracture zones lie at right angles to ridge segments.

Active slip (earthquakes) occurs between ridge segments.

Portions of fracture zones extending beyond ridges are notseismically active.

Fig. 2.22b, c

!

Inactive

f racture z

one

(No movement)

Inactive

f racture

zone

(No move

ment)

Active

transf orm

f ault

Fracture zon

e

Youngerplate

Mid-oceanridge

Olderplate

"

!

"




Point where three plate boundaries intersect.

Multiple boundary combinations occur.

Triple Junctions

Fig. 2.23a, b




Plumes of deep mantle material independent of plates.

Not linked to plate boundaries

Originates as a deep mantle plume

Plume partially melts lithosphere; magma rises to surface.

Hot Spots

Fig. 2.24




Hot Spots

Hot spots perforate overriding plates.

Volcanoes build above sea level.

Plate motion pulls volcano off plume.

Volcano goes extinct and erodes.

Chain of extinct volcanoes called

a hot-spot track .

Hot spots reinforce

sea-floor spreading.

Fig. 2.25b, d




Hot Spots

Hot-spot seamounts age away from originating hot spot.

Age trend defines rate of plate motion.

Line of seamounts indicates direction of plate motion.

Fig. 2.25a, c



Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. NortonChapter 2: The Way the Earth Works: Plate Tectonics

Continental lithosphere can break apart.

Lithosphere stretches and thins.

Brittle upper crust faults.

Ductile lower crust flows.

Asthenosphere rises and melts.

Magma erupts.

Continuation can create

a new mid-ocean ridge.

This process led to

the breakup of Pangaea.

Continental Rifting

Fig. 2.26a



Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. NortonChapter 2: The Way the Earth Works: Plate Tectonics

Continental Rifting

Western U.S. Basin and Range Province is a rift.

Narrow north-south mountains separated by basins.

Rifting tilted blocks of crust to form mountains.

Sediment eroded from blocks, filling adjacent basins.

Fig. 2.26b




Continental Rifting

East African Rift:

The Arabian plate is rifting from the African plate.

Rifting has progressed to sea-floor spreading in:

The Red Sea.

The Gulf of Aden.

Stretching continues along theEast African Rift.

Elongate trough bordered byfaulted high cliffs

Volcanoes – Mt. Kilimanjaro

The rift and two spreading ridges

comprise a triple junction.

Fig. 2.26c




Subduction consumes ocean basins.

Ocean closure ends in continental collision.

Subduction ceases, subducting plate detaches, sinks.

Continental crust is too buoyant to subduct.

Collision deforms crust, mountains are uplifted.

Plate Collision

Time 1: Before Time 2: AfterFig. 2.27a, b




Driving Mechanisms

Two forces drive plate motions:

Ridge-push—elevated MOR pushes lithosphere away.

Slab-pull—denser subducting plate is pulled downward.

Convection in the asthenosphere speeds or slows motion.

Fig. 2.28a, b

ch02 plate tectonics

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